migration: qemu_savevm_state_cleanup becomes mandatory operation

since commit
    commit 94f5a437
    Author: Liang Li <liang.z.li@intel.com>
    Date:   Mon Nov 2 15:37:00 2015 +0800

    migration: defer migration_end & blk_mig_cleanup

when actual .cleanup callbacks calling was removed from complete operations.

The patch fixes regression introduced by the commit above results in
100% reliable assert for virtio-scsi VM with iothreads enabled during
'virsh create-snapshot' operation:
    assert(i != mr->ioeventfd_nb);
    memory_region_del_eventfd
    virtio_pci_set_host_notifier_internal
    virtio_pci_set_host_notifier
    virtio_scsi_dataplane_start
    virtio_scsi_handle_cmd
    virtio_queue_notify_vq
    virtio_queue_host_notifier_read
    aio_dispatch

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Liang Li <liang.z.li@intel.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Juan Quintela <quintela@redhat.com>
CC: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>